Radiating and leaky coaxial cables are employed as longitudinal antennas in confined spaces like tunnels, mines, buildings and in other stretched-out applications involving a narrow lateral corridor needed for one or two way communication, such as railroads and highways.
Leaky coaxial cables support surface waves. The coupling loss between cable and antenna increases proportionally as 1/r.sup.2, where r is the distance between the cable and the antenna. The coupling loss also increases with increasing frequency. Leaky coaxial cables are known to employ, for example, outer conductors defining equally spaced groups of apertures, longitudinal apertures, corrugated outer conductors having milled-off corrugation tops, loosely braided outer conductors. Such prior art leaky coaxial cable designs are typically broad-banded.
Radiating coaxial cables radiate a free space wave. The coupling loss increases with 1/r, and is fairly constant over a relatively narrow design bandwidth. Prior art radiating coaxial cables are known to employ groups of apertures defined in the outer conductor. About half of the apertures within a group are tilted forward, the other half backward. The spaced groups of apertures must be designed about the center operational wavelength and to be configured for the specific bandwidth requirement.
U.S. Pat. No. 4,366,457 and corresponding German Pat. DE 30 04 882 C2 employ groups of coupling apertures which are so designed as to primarily support surface waves in comparison with radiating waves. The spacing between adjacent groups of coupling apertures is substantially larger than the operational wavelength, and more specifically is larger than 10 meters.
U.S. Pat. No. 5,276,413 describes a coaxial cable defining equally-spaced groups of apertures which excite a surface wave. There is one aperture per group in the first section at the cable input. The number of apertures within a group progressively increases with each subsequent cable section in order to maintain the coupling loss to be approximately constant along the length of the cable while the internal insertion loss increases. The group spacing must be smaller than half the smallest operational wavelength to avoid resonance return loss spikes within the operational frequency band. The number of apertures per cable length is therefore high, thus increasing the insertion loss and increasing the potential of moisture migrating through the apertures and into the cable.
U.S. Pat. No. 5,291,164 shows a coaxial cable which radiates a free space wave within the operational frequency band. The cable provides groups of apertures designed with respect to the center operational frequency. This design generates a surface wave below the radiating operational frequency band, but a high coupling loss limits its use at those frequencies. The bandwidth of the radiating operational frequency band is relatively narrow.